Transducer



July 8, 1952 w. M. WEBSTERQJR TRANSDUCER 2 SPEETS-Sl-IEET 1 Filed Sept. 20, 1950 560.60 930V: JEN/N6 $7710: 14

July 8, 1952 Filed Sept. 20, 1950 w. M. WEBSTER, J

TRANSDUCER 2 SHEETSSHEET 2 INVENTOFL William MWeZsiemb:

in Figure 4; a

Figure 11 is a schematic circuit diagram of 'a' sound reproducing system using the pick-up device of Figure 3; I.

Figure 12 is a schematic circuit diagram of a sound reproducing system using the pick-up device shown in Figures and 6; and

Figure 13 is a schematic circuit diagram of a sound reproducing system using the pick-up device shown in Figures .8 and 9.

The pick-up I0 shown in Figures 1 and 2 comprises a gas-tight envelope II- of substantially rectangular shape; Mostof the envelope II is made of rather. thickglass and .is quite rigid. However, its bottom I2 comprises a flexible, discshaped portion, (disc I3) consisting of a circular, relatively thin metal diaphragm formed with a number of concentric-corrugations and having its entire periphery fused to the edges of an opening in the glass part of thebottom.

A rigid stylus I4;.is.carried in, and extends and isvsealed through-the. center of the disc I3. In this arrangement; .anddueto the flexibility of. the-:disc, transversevibrations imparted to the ne le; pointifla on the-external end of the stylus l4 are imparted (with a reversalof direction) to its internal end. In. other'words. the center, (f) of the disc acts as a fulcrum for the stylus at a pointpintermediate its external and internal ends. 1

,There are two internal load current paths through pickup: I0. Eachof thepaths extends between a main cathode I5 and a respective one of pair of anodes I6, IT. A device for controlling the current between. the cathode I5 and the anodes I6, I! is hereinwdesignated. control grid I8, this name beingfused because of its function ratherthan-because of its structural appearance. The grid I8 includes a slot I9 which :is :partially blocked to'provide two mutually exclusivepaths for electron current from thelcathode I5 to the respective anodes I 6, I1. One oflthese paths extends through each of two long narrow openings-22. which are formed alongthe edges of the slot I97, by blocking the center of'the slot with a grid-shutter, 20,v The. long-axis of the shutter coincides with an upward extension of the axis of the stylus I4 on the internal end of which it is mounted. In the rest position of the stylus the long axis of the shutter also coincides with the long axis of the slot I9. In this arrangement a transverse vibration of the stylus will move the shutter 20- within the slot I9 so as to increase one of the openings 22 and at the same time decrease the other. It'will be apparent to those skilled in the art that this type of arrangement is appropriate for utilization'of this pick-up in a push-pull load circuit. As appears most clearly in Figure 2 they stationary portion of the grid I8 comprises a centralppartition 2| which shields each of theanodesj-l-t, .II from the other. In addition-it serves' to prevent electron current which passes through a given opening 22 toward one of the anodes It or I! from being collected on the other anode.

In the operation of this device both the stationary part of the grid I8 and its movable shutter 20 may be at the same potential. Because of this these parts may be electrically interconnected as shown at 26. The stationary portion of the grid I8 is supported on one of the end walls of envelope II by "a rod2'3. Rod 23 is shown to extend through this wall to the outside of the envelope to provide a terminal pin for connection to an external circuit element.

In the operation of pick-up III the electron current between the cathode I5 and either of the anodes I6, I! is through an extremely low impedance ionized gas plasma. In the novel gas valve employed herein, this plasma is established within the tube by ionizing its gaseous filling G by the action of an independent or auxiliary electron current flow. The auxiliary current flow is readily distinguishable from the load current flow through the tube. The following are two points of distinction: the auxiliary flow includes as one component a space discharge" electron current (as distinguished from conduction electron current) in which electrons attain suflicient velocities to ionize the gaseous filling, G, of the tube; and (2) the overall path of this space charge is different from that of the load current, e. g., it starts at an emissive electron source other than that at which the load current path starts this is the case herein where the two paths start at different cathodes, or it terminates at a collector other than the anode for the load current path, or it both starts and terminates at difierent electrodes than the internal load current path.

An auxiliary cathode. ismounted within the envelope II on the far side of the main cathode I5 from thegroup of elements including the anodes- I6, I! and'grid I8. It is surrounded by a cylindrical shield 25 having a narrow slot 21 on its. side toward the main cathode I5. The shield causes the ion-producing electron discharge to be formed as'a directed stream having a narrow cross section, 1. e., a constriction, in the region just beyond the point where the electrons emerge from the slot 21. .With this typeof construction a plasma of very high density. and therefore of very high conductivity, can be" provided by an auxiliary electron discharge of surprisingly low current, i. e., it can be economically provided.

The shield 25 is supported on a rod 28 which in the example herein is fused through an end of envelope II to serve as a terminal pin. This construction makes it possible to use a circuit in which one can control the bias between the auxiliary cathode 24' and theshield 25. This canbe useful as a way of electrically varying the, amount of constriction produced by the shield 25 without physically changing the width of its slot 21. ,However, it is entirely feasible to operate the auxiliary cathode and the shield at the same potential. Therefore,ffor some embodiments the shield 25 may simply be mounted by one or more rods which are secured to the interior of the envelope at suitable presses without necessarily extending any of the'rods through to the outside of the envelope. In'such an arrangement an internalinter connection may be used for causing the shield and the auxiliary cathode to have the same potential. Moreover, as indicated by extensive experimentation, it isf1 altso feasible simply to allow this shield to ca 0a n In order. not to include unnecessary detail in the drawing, and because the. cathodes which appear in the various figures. may be of any of a numberof suitable types. known in the art, none of these cathodes. isshown in section. In general, indirectly heated types of cathodes (particularly for main cathodes) are to be preferred for a number of reasons, e. g., their greater rigidity (than filamentary cathodes) and the resultant freedom from microphonics; the re-- duced likelihood of hum when A.-C. heater current is used; and the more copious emission of electrons.

. Each of the pick-up devices shown herein may be processed in any of a number of ways well known in the art to provide a gaseous filling within its envelope prior to sealing off. Any suitable gas or mixture of gases may be utilized. The gas pressure for a particular pick-up will be in accordance with its specific electrode geometry and spacings and must bev such as to favor the formation of a self-sustaining ionizing discharge. A number of gas tubes of the kinds employed in the present pick-ups have been found to operate satisfactorily with a filling of helium at a pressure of approximately 750 microns. However, as is well known other gases and other pressures may be used, c. g., gas pressures which lie within the range between approximately 180 microns and several millimeters of mercury.

ducer ofFigure-eover that of Figures 1, 2)

is in the structure of -its control grid [8a. The stationary portion of .grid [8a comprises a plurality of slots [to rather than a single one like the single slot E9 of grid l8. Accordingly the movable portion is a grid shutter 20a consisting of a plate which has the same number of similarly spacedslots and-which overlays the slottedfront of the stationary portion of thegrid [3a, rather than a narrow strip (like 20) which is inserted between the edges of a single slot (like !9). In the'rest' position of the stylus [4, i. e., when there is no force acting to deflect it transversely, each slot'in plate 201: will be aligned with one-half of the open area of a respective one of the slots l9a. As a result if the tube is built symmetrically and if the emission from the cathode is is substantially uniform all around its periphery, substantially equal currents will be received on the two anodes l5, ll. This is desirable in. circuits such ,as that shown in. Figure 10, in which the attainment of equal and opposite directcurrentcomponents in the push--' pull load circuit currents will obviate any necessity of mechanically or otherwise biasing the voice coil 3 I. It also tends to cancel certain noisecomponents such as anyGO cycle hum which may arise ,fromthe use of A.-C. heater current. .The groups of slots'located on oppositesi'des of the center axis of plate 20a uncover halves .of re.--

spective slots 19a which are oppositely positioned with respect'to the center lines of those .slots, i. e., which are positioned to, the right and left thereof. Asa result, a transverse displacement.

Figure 10 is a circuit diagram of a sound reproducing system employing either a pick-up device of the kind shown in Figures 1 and 2 or the modification thereof shown in Figure 4. The utilization device in this system is a transducer 38 having a-center-tapped voice coil 3|. Each of the anodes m ll-is connected to a different end of the voice coil-3i; The primary source of electrical energy for driving the voice coil 3! is a low voltage battery33. This battery may consist of a few large-size, serieseconnected one and one-half volt dry cells (as few as one or as many as'6 or -7) and it should be capable of delivering an average output of the order of at least ampere. The main cathode I5 is connected to the negative side of the battery 33, this connection being made, as shown herein, over a path through ground. In the. operation of this devicelarge currents of the order of one or more amperes may circulate through the load circuit even though the potential drop between either of the anodes it or H and the cathode I5 is no greater than one or two volts. With a hard tube that is not hundreds of times larger than this pick-up, this would not be at all possible. One-reason for this is that the emission from the cathode of a hard tube is space-chargelimited when'its controlgrid is biased for class of the plate: Zila'will increaseallopenings providing current pathsto one 4 of the anodes and simultaneously"decreaseall of those providing current paths to the other.

A operation. Moreover, suchcurrents would not even be possible in a gas'tube (despite its freedom from space-charge-lim-ited cathode emission) if it be of the prior art type in which the ionizing electron current moves between the same electrodes as the load current. This is due to the fact that thecurrentmust not exceed the value at which the IR drop in the external circuit will cause the applied voltage to be reduced below the value needed for sustaining ionization. Gas tubes-in which the ionizing currentmoves between the same electrodes as the load current have the additional shortcoming that a grid through-which those currents pass to reach ananode invariably loses control once the tube is ionized. 'The reasons for this have been understood for some time: the negatively biased grid gathers about itself a sheath of positive ionswhich determines the" efi'ective average or quiescent size ofthe grid openings; an instantaneous signal voltage superimposed on the bias varies this average size so that one might expect it to exerta' val'vi-ng-efiect on thecurrent; however, aneflect of this i's-that the constriction of "the stream of ionizing current, which exists whereever it'passes through a grid opening, also varies and this produces a variation in the plasma density which so opposes the change in the effective resistance of the grid that a substantially' ."'-constantcurrent continues to flow regardless ofthe signal voltage'variations. In other words, as =achange .inthe cross sectional areafofthe plasma in. the plane of the-grid varies its resistance in a region near to the grid, a change occurs inits density which oppositely varies its resistance. Instead in a gas tube which operates on the principle of the device shown hereinthe plasma is-produced by an auxiliary source so that its density is not 'so influenced by variations in the thickness ofan ion sheath around agrid for controlling load current. Therefore, "an efiective variation in the size of a*control; grid open'ing will actually cause 'a change in'the load-currentfi It willdo so since there will be nothing to simultaneouslyinfiuence the plasma-density"sofas to producewan equal and opposite change fln :its. overall conductivity 1 in the load current path. In view of the foregoing it is obvious that mechanical variations in the grid openings of the devices of Figures 1 and 2 and 4, though they may have some action akin to constricting, the load current where it passes through the grid openings, will not have any general effect upon the plasma density which will offset the valving action of the grid. Any electron discharge which accompanies the electron oonduction in the load current flow will be at too low a velocity to contribute to the production of plasma and therefore its variations will not affect the plasma density. The grids 18 or lSa shown herein are effective control devices because mechanical variations in the size of their load-current carrying openings vary the useful cross section of the conductive plasma in the plane of those openings, 1. e., vary the cross sections of the corridors of plasma which extend through them.

A further disclosure of principles governing the operation of a gas tube of this type is to be found in a coy-pending application of E. 0. Johnson, Serial No. 185,745, filed September 20, 1950. From the foregoing, it is apparent that the control grid should be biased at a somewhat negative direct potential if it is to be responsive to an externally applied signal voltage. This is because the effectiveness of the signal depends on the existance of an ion sheath which it can control to vary the effective size of the grid openings. Since they plasma extends through the openings its, conductivity in the region of the grid will therefore be controllable by variations in the potential, thereof. In the devices shown in Figures 1, 2 and 4 herein substantially the same thing is accomplished by, simply mechanically altering the actual physical size of the grid openings through which the plasma extends to the anodes. In other words a grid of the kind shown herein is able to control the load current without dependence on the formation of a sheath, and therefore does not need to be negatively biased. Accordingly, it may be operated at any of a variety of positive potentials at none of which it will be surrounded by a positive ion sheath. On the other. hand, a variable negative bias may be applied to the grid, as shown at 40 in Figure 10, asa way of providing a control parameter. Since the existence of this bias will cause a sheath to form and its magnitude will determine the thickness. Therefore, this arrangement provides a way of adjusting the size of the load-current-carrying openings through the grid when the stylus'is at rest and may be used as a gain control. The auxiliary electron current in either the Figs. 1, 2 pick-up or in that of Figure 4 flows from the auxiliary cathode 24 to the entire group of the elements constituting its load current carrying-and-valving portion. Accordingly,- a circuit employing the pick-up should include a source of direct potential 34 by which the auxiliary cathode is polarized far enough below the potential oi at least one of theseelements to sustain an ionizing electron discharge between the auxiliary cathode and that element;

As previouslymentionedthe constriction of the ionizing current caused by the use of the slottedshield 25 makes it possible to maintain a relatively large potential drop across the auxiliary discharge. -Because-of this the electrons will attain ionizing velocitieswhen they have traversed only a relatively small'portion or their paths of. travel. Since they willjcontinue to produce ionsv as theyp'move rover the rest of their paths a relatively very dense plasma will be produced for a given magnitude oi! ionizing current. This plasma will bathe the main cathode l5 and extend through the grid openings to the anodes l6, l1 thereby neutralizing the electron space charge of the main cathode and providing a highly conductive medium in the load current path of the tube. It is advantageous to use grid openings which are large enough so that some electrons of the ionizing discharge current will be projected through them to produce plasma directly in the space between the grid and each anode and/or so that plasma may freely difiuse thereinto from the space around the main cathode. In fact, for best operation of any of these pick-ups, the plasma should extend continuously from the main cathode(s) to the main anode(s).

It will be apparent, from the fact that variations of conductivity are efiected by varying the cross sectional area of the plasma rather than by varying its density, that the frequency response of this pick-up does not have an upper limit which is a function of deionization time. Actual tests indicate that this type of device has a lfrequency response which far exceeds the limits required for high fidelity audio reproduction.

The embodiment of Figure 3 is intended for simple class A operation, for example, in a circuit like that of Figure 4, rather than for pushpull operation. To this end it is a modification of the Figure 4 embodiment in that only a single anode 29 is used and the structure of its grid i812 is simplified accordingly.

The embodiment shown in Figures 5 and 6 differs from all of the others thus far described in that a different type of grid control is employed. The control grid 35 for this embodiment does not consist of two portions, one stationary and one movable. Instead the entire grid is a unitary structure and movable as such. This structure comprises a base plate 35 of an approximately triangular shape which is carried on the top end of the stylus M as it appears in Figure 6. A row of grid wires 31 extends upward from the top surface of the plate 36 along each of two of its sides. Each row of wires is parallel and closely adjacent to one of the anodes 16a and Ila.

This embodiment normally requires that the grid be negatively biased so that a sheath of ions surrounds 'each of the grid wires. This is best understood with reference to Figure 7. Figure 7 shows how the conductive corridor (of plasma) 38 which extends to an anode (Ila) between the sheaths 39 around two adjacent grid wires varies in length as the grid wires are moved with respect to theadjacent anode. Though the plasma is a highly conductive medium, it does have a finite resistance and the value of that resistance for each corridor 38 is directly proportional to its length. Therefore, the resistance of the plasma to a given anodewill increase as the grid is moved away from it. I

A circuit which'is suitable for the Figures 5 and 6 embodiment is shown in Figure 12. The operation of this circuit will be readily understood in view of the'foregoing explanation of the circuit shown in Fig. 1.

The Figsg8, 9 embodiment includes a gas valve which operates inia somewhat different way to control electron conduction through the load current path'ofrthe .pick.-up.. In this embodiment the means for jmodulating the over-all conductivity ofqthe. plasma operates by varyin its density over most or all oi'the load: current path rather than byvarying its useful cross-sectional area in a small region of said path near the plane of a control grid. While this necessarily means that the frequency response-of this pick-up will be limited at its upper end by de-lonization time, nevertheless, the useful frequency range will be more than adequate to cover the entire audio frequency range. A further disclosure of principles governing the operation of a gas tube of this type is to be found in a co-pending application of William M. Webster, Jr. Serial No. 197,014, filed November 22, 1950. As will be best understood with reference to Figure 9, certain elements of this embodiment which are related to the function of producing the ionizing electron discharge include what are essentially the cathode, control grid, and anode of what may be called-an auxiliary triode. More specifically these elements are: the auxiliary cathode 24; -a -Inovable control grid ii; and a slotted anode 42. In the operation of the tube, the potential of theslotted anode 42 is positive with respect to the auxiliary cathode 24 but its magnitude is less than that required to produce ionization.

Some of the electrons which are attracted from the auxiliary cathode 24 towardthe slotted anode 42 will be projected through its slot '43 and upon emerging from its will move toward a group of electrodes including those which carry the useful load currents within the pick-up. Since the auxiliary triode is operatedwith such a low anode potential, it will behave like a vacuum tube rather than a gas tube, i. e.,-its grid can be used to effect plate current variations in a substantially linear fashion over a usefully large range. It can be made to do this either by keeping the mu constant and changing the grid voltage or by doing the converse. As is well known the mu of a triode can be varied by changing the spacing between its cathode and control grid. This principle underlies the arrangement used herein. In this pick-up the stylus I4 is connected to the underside of a base plate M of the control grid 4! in such a manner that a transverse movement thereof will tilt the base plate and will thereby alter the grid-cathode spacing between a U- shaped row of grid wires 45 (which extend upward from the base plate) and one side of the auxiliary cathode. Thus-with an appropriate value of grid bias between the grid and this cathode, the current of the electron discharge in this triode section can be modulated by mechanical movements of the stylus. Asa result the magnitude of the current of the portion of this discharge which escapes through the slot in the anode will likewise vary and'this, in turn, will directly control the production of plasma in the right hand portion of the tube,

In the arrangement shown herein, it is desirable to prevent or inhibit any tendency 'of positive ions to drift from the right hand region of the tube back through the slot 43 into the interelectrode spaces of the auxiliary triode where their presence would tend to impair its proper operation. Accordingly, means are provided herein for trapping positive ions in two low potential troughs from which it will be difficult for them to escape so as to be free to move toward the triode section. A first low potential trough may be established just beyond the slotted anod'e .42 by properly polarizing a first screen grid 46 which extends across the path of the electrons which emerge therefrom. In the operation of this device, this screen is maintained at a potential above that of the auxiliary-cathode but below ions.

that'of the slotted anode, As a result of the electrons which, aseX-plained above, will emerge from the slot at velocities too low-to produce ionization, will be retarded (but not stopped) so as to emerge from the right hand side of the screenat even lower velocities. The next element inthe path of these electrons is an accelerating screen 47. In the normal operation of the pick-up, ,this screen is maintained at a potential somewhat above the ionizing potential of its gaseous filling G. In this arrangement it is to be expected that at leastsomeofthe electrons approaching the accelerating screen t! will have ionizing collisions slightly before they reach it andv that for this reason positive ions will be present to the left of this screen (as it appears in the drawing). -I-I0wever, leftward movement of these ions will be limited as most of them-will be attractedinto and trapped within the low potential trough of the first screen -grid-45.. Of course, the inertia of some of these ions wil lbe sufficient so as to tend to carry them all the way upthe left slope of the potential trough. However,;most;of these will lose much of their inertia incollision with slow Moreover, most of those which do get up the left slope will collide with solid portions of the slotted anode 42 so that only a fewwill pass through its slot 43.; 'I heslowerionswill oscillate leftward and rightward through the -fl-rst screen grid at thereby remaining trapped in the trough which it establishes and eventually they will be collected and neutralized on the-first screen grid to. Most of the ionizing collisions will occur in a second low potentialtrough to the right of the accelerating screen 4-1 in which, therefore, a dense plasma willnbe established. It is in this region that the main source of thermally emitted.

electrons is located, This source in the present example is a pair, of indirectly heated cathodes 58. The main anode 13-9 is mounted tothe right of these cathodes. It may be polarized atv the same potential as theaccelerating screen i'lwhile the two cathodes stare polarized at aipotential a few. volts below it, i. e., below the common potential for these two elements. In this wa a low potential trough will be established between the accelcrating screen t! and the main anode 49 in which trough positive ions will tend to .be trapped. This is the load current carrying portion of the pick-up. vThe loadscircuit battery 33, which is used for maintaining the small potential difference between the main source of electrons (the pair of cathodes 48), and the accelerating screen t! and the main anode .49, respectively, should be a high current source, such as a number of seriesparallel-connected 1% volt dry cells, a storage battery, or the like, since it must serve notmerely for establishing a bias but also for delivering to the utilization device a signal-bearing current having an average value of the order ofv one to several amperes. I In this embodiment the plasma not only has the impedance-lowering effects described above, including, of course, that of neutralizing the electron space charge of the main cathodes. In addition it is; variable, under control. of the auxiliary triode to have an impede ance-modulating; effect on which the valving If desired, the control grid 4| may be eliminated from the "auxiliary triode by using an arrangement in which movements of the stylus I4 are caused to vary the spacing between the auxiliary cathode 24 and the slotted anode 42, e. g., by moving either one of these elements while the other is held stationary.

The Figs. 8, 9 pick-up may be modified to produce an embodiment which is suitable for pushpull operation by providing on the left hand side of the auxiliary cathode 24 a duplicate set of the electrodes which are shown on its right in the drawing. As a result the pick-up would comprise two auxiliary triodes each having its own slotted anode but both having the same cathode and control grid. In such an embodiment, a portion of the control grid would lie to the left of the auxiliary cathode 24 just as a portion of the grid 4| lies to its right as shown in Figs. 8 and 9 as a result each transverse movement of the grid would increase the current passed by one of the auxiliary triodes and at the same time decrease that passed by the other.

Figure 13 shows a circuit which is suitable for using the Figs. 8, 9 pick-up device as explained above.

I claim:

1. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a group of load circuit electrodes including at least a main cathode and a main anode having, respectively, electron-emitting and electron-receiving surfaces which define the opposite ends of a predetermined load current path; means for producing an ionizing discharge of electrons along a different path within said envelope to provide a conductive plasma which is continuous from one end to the other of said load current path; a flexible portion for said envelope; and means responsive to mechanical forces exerted externally of said envelope and communicated to its interior over said flexible portion for varying the over-all conduction provided in the load current path by said plasma.

2. A transducer as in claim 1 in which said last mentioned means comprises a means responsive to said forces to vary the over-all density of said plasma to vary said conduction provided by it.

3. A transducer as in claim 1 in which said last mentioned means comprises a means responsive to said forces to vary in a predetermined region along said load current path the cross-sectional area of the plasma usefully available for conduction of load currents to thereby vary the over-all conduction provided in the load current path by the plasma.

4. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a group of load circuit electrodes including at least a main cathode and a main anode having, respectively, electron-emitting and electron-receiving surfaces which define the opposite ends of a predetermined load current path; means for producing an ionizing discharge of electrons along a different path within said envelope to provide a conductive plasma which is continuous from one end to the other of said load current path; a flexible portion for said envelope; said group of load circuit electrodes also including a movable element comprising a control grid havin openings which are in alignment with the load current path and are large with respect to the mean free path of a positive ion, said grid being in close spaced relationship to the main anode; and means linking said movable element to the inside of said flexible portion for varying the closeness of said spaced relationship in response to mechanicalforces exerted externally of said envelope, whereby said forces will be effective to vary the lengths of plasma corridors extending through the'grid openings to the main anode.

5. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a group of load circuit electrodes includin at least a main cathode and a main anode having, respectively, electron-emitting and electron-receiving surfaces which define the opposite ends of a predetermined load current path; means for producing an ionizing discharge of electrons along a different path within said envelope to provide a conductive plasma which is continuous from one end to the other of said load current path; a fiexible portion for said envelope; said group of load circuit electrodes also including a control grid having at least one slot aligned with the load current path of said main anode, said grid includin a movable element positioned effectively to define a perimeter of the slot, and said movable element being linked to the interior of said flexible portion of the envelope to act as a shutter for varying the cross-sectional area of the plasma where it extends through the slot in response to mechanical forces exerted externally of said envelope and communicated to its interior from said flexible portion.

6. A transducer as in claim 5 in which said control grid surrounds said main anode and except where it is slotted constitutes a continuous shield between it and other electrodes of said transducer.

7. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a group of load circuit electrodes including at least a main cathode and a main anode having. respectively, electron-emitting and electron-receiving surfaces which define the opposite ends of a predetermined load current path; means for producing an ionizing discharge of electrons along a different path within said envelope to provide a conductive plasma which is continuous from one end to the other of said load current path, said means including as two of its elements an emissive source of ionizing electrons and a collector thereof, at most only one of two said elements being included in said group of load circuit electrodes; a flexible portion for said envelope; a lever sealed through said flexible portion to extend from the interior of said envelope to its exterior whereby a movement imparted to the external portion of the lever will be reproduced at its internal portion; and means within the envelope including a movable element which is mechanically linked to the internal portion of said lever for responding to movements thereof to vary the overall conduction provided in the load current path by the plasma.

8. A transducer as in claim 7 in which said emissive source of ionizing electrons is an auxiliary cathode and said collector comprises one of said group of electrodes such as said main cathode.

9. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a group of load circuit electrodes including at least a main cathode and a main anode having, respectively, electron-emitting and electron-receiving surfaces which define the opposite ends of a predetermined load current path; means for producing an ionizing discharge of electrons along a different path within said envelope to provide a conductive plasma which is continuous from one end to the other of said load current path; a flexible portion for said envelope; and means responsive to mechanical forces exerted externally of said envelope and communicated to its interior over said flexible portion for modulating the ionizing discharge to correspondingly modulate the density of said plasma.

10. A transducer as in claim 9 in which said means for producing an ionizing discharge includes an auxiliary cathode as a source of ionizing electrons and at least one of said group of load circuit electrodes as a collector thereof; and said last mentioned means comprises a control grid movably supported in close and variable space relationship to the auxiliary cathode on its side toward said group of electrodes and linked to said flexible portion for control of said relationship in accordance with said-mechanical forces.

11, A transducer as in claim 10 in which said main anode is on one side of said main cathode and said auxiliary cathode and said control grid are on the other and which further comprises means, including at least one electron permeable screen positioned athwart the path of the ionizing discharge between the auxiliary cathode and the control grid, for establishing a low potential ion-trapping trough in a region near said main cathode whereby the concentration thereof will be much greater in said load current path than in the region between said auxiliary cathode and said control grid.

12. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a main cathode and a main anode having surfaces defining the opposite ends of a predetermined load current path; means for producing an ionizing discharge of electrons along a diiferent path within said envelope to provide a continuous plasma over all of said load current path; a control grid for blocking the movement of electrons over some of the lines of travel, between the main cathode and the main anode, over which such travel would otherwise be possible, but having at least one opening so that it does not block their movement over others of said lines of travel; a flexible portion for said envelope; said control grid comprising a movable shutter-portion, linked to the interior of said flexible portion of the envelope and positioned to define a perimeter of said opening, for responding to externally exerted mechanical forces imparted. to it over said flexible portion to vary in the cross-sectional area of said plasma where it extends through the opening.

13. A sound reproducing system comprising: a transducer including a sealed envelope containing a gaseous filling, within the envelope a main anode and a main cathode having surfaces defining the ends of a predetermined load current path, and means including as two of its elements an emissive source of ionizing electrons and a collector thereof for producing an ionizing discharge of electrons along a different path within said envelope to provide a conductive plasma which is continuous from end to end of said load current path, and means responsive to mechanical forces exerted externally of said envelope for varying the over-all conduction provided in the load current path by the plasma; a low impedance reproducer; a load circuit including in series said reproducer and a high current source of electrical energy, for applying between the main anode and main cathode a potential of insufiicient magnitude to ionize said gaseous filling; and an ionizing circuit for applying between said two elements a potential of sufiicient magnitude to ionize said gaseous filling.

WILLIAM M. WEBSTER, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,507,884 Engler Sept. 9, 1924 1,871,253 Bauer Aug. 9, 1933 

